1. Field of the Invention
The present invention relates to the field of containers for dispensing fluids under pressure. More particularly, the invention relates to a plastic container for dispensing fluids under pressure which is adapted to employ an expandable bag to generate the expulsion pressure within the container, and the method for manufacturing such a container.
2. Background of the Invention
Containers for dispensing fluids under pressure have traditionally fit into two catagories: (1) pressurized containers, such as the common aerosol container, in which constant pressure is exerted on the fluid to be expelled both during use and non-use; and (2) pump-type containers in which the user creates the expulsion pressure during use by manual actuation of a pump apparatus. Commercially suitable pumptype containers have been constructed from a variety of materials, including metals, glass, ceramics and plastics, among others. Since the expulsion pressure is only developed during times of actual use in these containers, complete gas impermeability has not been a design requirement.
In contrast, commercially available pressurized containers have almost exclusively been constructed of metal. By and large, gas propellants within the container have provided the requisite pressure for expelling the fluid contents of the container upon actuation of a valve mechanism. Gas impermeability has therefore been a prime requirement for such containers, and since techniques for forming gas impermeable seals in metal containers have long been successful, metal containers have prevailed.
Metal dispensing containers do possess disadvantages, however, not the least of which are high material costs and manufacturing complexity. Further, such metal containers generally employ mounting cups in which the valve assembly is mounted and which must be crimped to the top portion of the container. Consequently, efforts have been made to produce pressurized dispensing containers made from other materials, especially plastics. To date, however, these efforts have not resulted in an entirely satisfactory alternative to the pressurized metal container.
For example, U.S. Pat. No. 3,140,802 discloses a pressurized dispensing container which includes a collapsible bag and a separate compartment into which an expandable fluid, such as "Freon" is placed. The bag is seated securely in a container which may be constructed of plastic, and attached to the side wall of the container by an adhesive or by heat sealings. (Col. 3, lines 4-7). The lower surface of the bag is adjacent to a "cradle" which forms one wall of the Freon-containing compartment. Holes are formed in the cradle so that the expanded gas can exert pressure on the exterior surface of the bag and cause its contents to be expelled when the valve of the container is actuated. This arrangement represents an attempt to separate the fluid contents to be expelled from the container from the pressure-producing gas propellant and thus reduce the number of sealing locations where the gas can escape.
The seals between the bag and the interior wall of the container and the lower portion of the container thus become critical in assuring that the gas propellant does not escape into the atmosphere.
U.S. Pat. Nos. 3,698,595 and 3,791,557 are representative of an alternative approach to the solution of the gas permeability problems which have heretofore been endemic to pressurized plastic dispensing containers. Both of these patents disclose pressurized dispensing containers in which the pressure is developed on the fluid to be expelled by the contraction of an elastic bladder in which the fluid is contained. the former patent states that one of the advantages of a bladder-type container such as the one disclosed therein is that the container assembly can be made of plastic. (Col. 1, lines 34-38). Bladder-type containers thus solve the gas permeability problem by eliminating gas as a propellant altogether. Quite obviously, the structural integrity and contracting characteristics of the bladder material are limiting factors on the suitably of this type of container for commercial use.
Lastly, British patent publication No. 2,101,225 discloses a plastic dispensing container which, like the common metal aerosol container, uses a gas propellant mixed in with the fluid to be expelled to create the requisite expulsion pressure. In this case, an attempt is made to solve the gas permeability problem by a variety of sealing arrangements between the valve assembly portion of the container and the vessel portion. A metal valve cup is used in the preferred embodiment and sealing is accomplished by crimping the metal over the plastic rim or "bead" of the vessel portion of the container.